1. Field of the Invention
The present invention relates to the preparation of hexene-1,6-dioic acids, in particular hex-3-ene-1,6-dioic acid.
2. Description of the Prior Art
It is known to this art that hex-3-ene-1,6-dioic acid is readily hydrogenated into adipic acid.
Adipic acid, one of the basic starting materials for nylon 66, is currently produced in vast amounts, and, for this reason alone, any novel process providing access to such diacid and/or derivative thereof would be of significant interest to this art.
Continuing, U.S. Pat. No. 4,611,082 indicates that it is virtually impossible to carbonylate a solution of 1,4-diacetoxy-but-2-ene in a polar and non-basic aprotic solvent, selected from among the nitriles, bis(2-methoxy)but-2ene, bis(2-methoxyethyl) ether and methylene chloride, at 80.degree. C. to 140.degree. C. in the presence of a transition metal halide, and that, in the presence of an alcohol, the rates increase and are comparable to those determined for the carbonylation of but-2-ene-1,4-diol. With regard to this latter substrate, it is also indicated that it is not possible to achieve satisfactory yields of straight-chain carbonylation products under the aforementioned conditions and, in this context, preference is given to the substrates substituted in the 1,4-position by alkoxy groups.
Also in this regard, it would therefore appear that 1,4-diacetoxy-but-2-ene cannot be regarded as a promising substrate for forming dicarbonylated straight-chain products.
However, 1,4-diacetoxy-but-2-ene is readily available by acetoxylation of butadiene. It would therefore be highly desirable to provide a process enabling dicarbonylated straight-chain compounds to be prepared with a high efficiency from 1,4-diacetoxy-but-2-ene, for example, and more generally from butenes disubstituted by acyloxy groups.